It is well known that the low noise amplifier is one of the essential components among wireless RF system devices. Commonly a low noise amplifier is used at the rear end of a wireless receiving device (e.g. antenna), to amplify very weak signal(s) received by the receiving device. The low noise amplifier requires a circuitry with low noise and high amplified ratio. Also when the input signal is strong it requires a more precise linearity and low gain.
FIG. 1 shows a block diagram of a prior art low noise amplifier having a switch parallel connected to the amplifier. The switch forms a bias circuit. The switch turns on when the input signal is low, forming an amplified gain. The switch turns off when the input signal is high, ensuring system linearity. However since a 50 Ω impedance is required in the system, energy consumption is high, and moreover the whole system cannot be integrated. Even when CMOS technique is used, a larger potential capacitor will be required, but with low isolation, which will induce a mismatched input signal when the switch turns off.